We usually think of asteroid impacts as harbingers of mass extinction, but they might be the reason life exists on our planet at all. It's possible for bacteria to hitch rides on rocks ejected by space impacts and move from planet to planet. We know that certain Earth bacteria are capable of surviving the hostile conditions of space, but could they survive the impacts themselves? A group of scientists put them to the test, and we've got the results for you.The theory that life can be carried between planets on rocks ejected by impacts is called lithopanspermia. Some bacteria are able to survive the cold, the dryness and the radiation present in space, especially if they're buried deep inside large asteroids (like Eros, pictured). The shock pressures produced by the impacts that would eject them into space in the first place have always been a stumbling block for lithopanspermists (they love it when you call them that). A research team made up of German, Russian and U.S. scientists put bacteria thought to have the potential for successful space travel through some rough treatment, using explosives and air guns to slam them around in metal containers. The shock pressures used were similar to the pressures experienced by Martian rocks that were ejected by impacts and eventually reached Earth - we've found about 40 of them so far. The results: those bacteria are pretty tough. Bacterial endospores and lichens could handle very robust shock pressures, while cyanobacteria were more fragile, but still capable of withstanding impacts strong enough for interplanetary travel. Going from Mars to Earth is a definite possibility. So if the Phoenix Lander finds evidence of past life on Mars, it could mean that all life on Earth is alien. Image by: NASA.Microbial Rock Inhabitants Survive Hypervelocity Impacts on Mars-Like Host Planets: First Phase of Lithopanspermia Experimentally Tested. [Astrobiology]